Structural analysis of crystalline and nanocrystalline materials

The structure, atomic and crystallographic, as well as the microstructure of a material, determine the physical properties of the material.  The characterization of a material is therefore of considerable importance.  A solid is an aggregate of randomly oriented grains of crystal.  A crystal grain is a regular array of one or more types of atoms. The disruption of the arrangement of atoms in a crystal results in a crystal defect.  Material characterization involves the identification of the atoms, their crystallographic structure and the microstructure of the target solid.  An element is usually identified by its characteristic X-ray spectrum.  Crystal structure is often determined by X-ray or electron diffraction. The characterization of a small grain, especially nanocrystalline grain, requires a probe of nanometer size.  The transmission electron microscope (TEM) is the only instrument capable of providing chemical and structural (atomic and crystallographic) data for the characterization of a solid with high spatial resolution.  By focusing a nanometer probe of electrons onto a small grain in a TEM, characteristic X-ray and diffraction pattern are generated.  The detection and analysis of X-ray by means of Energy Dispersive Spectroscopy (EDS) in a TEM allows the identity and amount of the elements in the solid to be determined.  Electron diffraction provides a means for the identification and determination of crystal orientation and structure.  Imaging, the ultimate goal of microscopy, can be carried out in different ways depending on the occasion. Diffraction contrast imaging is usually used for the determination of crystal defects. High resolution TEM imaging is used for the observation of the atomic arrangement in the crystal.

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Magnetic Materials

Novel Properties of Metallic Nanoparticles and composites

Giant Hall Effect from embedded nanoparticles

At the Physics Department of HKUST, a group of scientists have discovered a large increase in Hall response in magnetic metal/insulator nanocomposites, Fe-SiO2, Ni-SiOand Co-SiO2, when the metal concentration is just enough to form a connected network (the percolation threshold). It has been also demonstrated by studying the non-magnetic Cu-SiO2nanocomposites that the Giant Hall effect can be ascribed to the local quantum interference effect (Phys. Rev. Lett. 86, 5562 (2001)).


Fig. 1
Fig. 2

Oxidation/corrosion resistant nanoparticles of Fe and their ferrofluid

Passivated nanocrystals of iron prepared by gas condensation of plasma evaporated vapor in Tianjin University are remarkably resistant to further oxidation and corrosion. We have shown by electron diffraction and high resolution transmission electron microscopy that each nanocrystal of Fe is enclosed by a shell of single crystal-like epitaxial gamma-Fe2O3 oxide (Fig 1 and Fig. 2). The passive oxide shell is about 4 nm thick. This is thick enough to provide effective protection to the metal core at room temperature (Appl. Phys. Lett. 77, 3971 (2000)).

Ferrofluid has been fabricated with these corrosion resistant nanocrystals of iron. As shown in Fig. 3, when the ferrofluid is subjected to the non-uniform magnetic field of a NdFeB permanent magnet, it transforms into a solid with long and sharp spikes.


Fig. 3

Discovery of magnetic refrigeration materials

Anisotropic magnetocaloric effects have been observed experimentally for the first time by using the identical anisotropic magnetic Fe8 clusters, which has been intensively studied for the Resonant Quantum tunnelling of Spins. It is also interesting to examine the anisotropy effect using this model system, because the nano-structured magnetic materials usually show a strong magnetic anisotropy (Phys. Rev. Lett. 87, 157203 (2001). Due to the simple form of the Hamiltonian in the system, the dynamics of the magnetic moments can be strictly obtained using quantum mechanics. The quantum mechanical solution agrees excellently with the experimental results.

Rare-earth based, inter-metallic alloys have been synthesized and characterized for the search of magnetic refrigeration materials used for Active Magnetic Regenerator Magnetic Refrigerator (AMR). LaFe12-xSix alloys with a NaZn13 type cubic structure have been discovered to be a good material candidate for AMR in the temperature range -70 degrees Celsius to room temperature (Appl. Phys. Lett. 77, 3072 (2000)).

Carbon Nanotubes: superconductivity in 0.4 nm single-walled carbon nanotubes

At temperatures below  15 \ \text{K} , the Meissner effect, superconducting gap and fluctuation supercurrent have been observed in 0.4 nm carbon nanotube / zeolite composites. The measured superconducting behaviors display smooth temperature variation due to fluctuations, with a mean-field  T_c = 15 \ \text{K} .

The three separately measured superconducting behaviors can be consistently explained within a unified theoretical framework.  The three effects are separately explained below.

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Electrorheological (ER) fluids

Electrorheological (ER) fluids denote a class of materials consisting of nanometer to micrometer sized solid particles dispersed in a liquid, whose rheological (i.e., deformation and flow) properties are controllable by an external electric field. In particular, they can reversibly transform from a liquid to a solid within one hundredth of a second. While in the solid state (with the electric field applied), the strength of that solid, measured by the yield stress, is the critical parameter that governs the application potential of the ER fluid. Since their discovery some sixty years ago, there has been much effort in searching for ER fluids with high yield stress, mainly due to their applications in vibration damping, clutches, and practically all mechanical devices that involve motion transmission. In particular, the automotive applications, as a potential replacement for gears, have attracted sustained research at all the research laboratories of major automotive companies. In the late 1980s General Motors carried out a study on the application potential of ER fluids, and concluded that a major hurdle was the low yield stress of the ER fluids.

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Carbon Nanotubes: fabrication of world’s smallest single-walled carbon nanotubes

Using microporous zeolite single crystals as hosts, Dr Zikang Tang and Dr Ning Wang from the Physics Department succeeded in fabricating the world’s smallest single-walled carbon nanotubes (SWNTs) that are periodically aligned in the crystal channels. The diameter of the single-walled carbon nanotube is only 0.4nm (nanometer).

Carbon nanotubes are long, thin cylinders of carbon. Single-walled carbon nanotubes are formed by rolling single-atomic graphite layer into a cylinder. They are extremely small in size – a bundle of 1,000,000 carbon nanotubes equals the size of a hair. Their unique physical structures, electronic properties, and their intriguing potentials for wide applications have sparked an explosion of research into their understanding since they were discovered in 1991.

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